Genes encoding MHC class I-like, class II-like and CD1 molecules have evolved to assume specific immunological functions. Some class I-like molecules, including H-2M3 and Qa-2, present formylated bacterial peptides or have distinct peptide-binding motifs. The class II-like DMA and DMB gene products play a role in presentation of peptide antigen by class II molecules. By contrast, CD1 molecules appear to have evolved separately into presenters of nonprotein antigens and into TCR ligands with specialized roles in the immune response. Thus, class I-like, class II-like and CD1 molecules appear either to serve important independent functions or to complement MHC class I and class II. It is expected that future efforts will increasingly reveal the functional ramifications of these molecules.